Flexible couplings or universal joints or more traditional Oldham couplings are typically used to interconnect opposing rotary shafts in photocopiers and other small business machines and instruments. The purpose of the coupling is to couple a motor to a driven device and to compensate for a limited amount of offset and/or angular misalignment between the shafts being connected. The compensation is performed in a universal joint by allowing rotational movement between the coupling members. However, because the coupling members are interlocked and cannot slide, universal joints cannot be used to interconnect shafts that are offset.
On the other hand, flexible couplings are most useful for interconnecting shafts that are angularly misaligned, as offset between the shafts causes strain on the coupling that can lead to fatigue and failure.
Non-interlocking couplings, for example Oldham couplings, are not useful because they are axially separable and thus may come apart during use, necessitating machine shut-down and coupling repair or replacement. In addition, Oldham couplings do not pivot and so cannot be used to compensate for angularly misaligned shafts.
These traditional shaft couplings are commonly made of two hub members, one being attached to each of the rotary shafts, and a center member which interconnects the two hub members. The hub members are normally made of metal. This provides strength, but adds to the cost because the metal must be machined. In addition, the hub member shaft hole is typically a round, drilled hole which may not exactly fit a variety of different shaft shapes and sizes. To accommodate these different shafts, the hub members typically include one or more set screws communicating with the shaft hole for ensuring a non-slip fit. Another problem with these metal couplings is that they transmit vibration from the motor to the driven device and also generate a relatively large amount of noise.
The center member of these traditional shaft couplings is often a cross piece which engages but does not lock with the hub members. These cross pieces are typically sized to fit relatively loosely in the hub members to allow for a small amount of angular misalignment or offset between the two opposing shafts.
Some improved center members are made of rubber. Due to their flexing, these rubber members provide some limited compensation for angular misalignment between shafts. They also cut down on transmitted vibration and noise. However, the soft rubber is subject to wear, which causes loosening and eventually leads to backlash problems. If the rubber pieces are made oversized or installed tightly to prevent these problems, they can lose some of their elasticity and create the noise and vibration problems inherent in the metallic center members.